It is known that, in order to produce bars, wires or other metal shaped pieces, particularly in non-ferrous metal, a metal billet is generally extruded from an extrusion press comprising a container delimiting a tubular housing centered on an extrusion axis, closed at its ends by two closing pieces, and into which a metal billet to be extruded is placed. One of the crosspieces has a die. The assembly is placed in a frame comprising two fixed crosspieces held apart from each other by tie rods. One of the closing pieces comprises a block of the same diameter as the housing, placed at the end of the ram. The block is introduced into the housing by a main extrusion jack which bears on a fixed crosspiece, and approaches the other closing piece bearing on the other crosspiece. The metal compressed between the two closing pieces is extruded through the die in the form of a shaped piece having the same section.
In the process known as "forward extrusion", the container is fixed and bears directly on one of the fixed crosspieces, the die being carried by the closing piece. The ram carrying the block constituting the other closing piece is driven by the main extrusion jack which makes the block penetrate into the housing of the container.
In the process known as "backward extrusion", the die is mounted on a block of the same diameter as that of the housing of the container, and placed at the end of a tubular ram which bears on one of the fixed crosspieces. The opposite end of the container is closed by a bottom, the assembly being mounted on a movable crosspiece driven by the main extrusion jack, such that the housing slides onto the block, causing the metal billet contained inside the housing to be extruded through the die, the extruded shaped piece being expelled through the tubular ram.
Metal billets are produced from long blooms which are cut up into lengths slightly shorter than the length of the housing of the container. The transversal section of each bloom is also slightly smaller than that of the housing of the container so as provide the clearance needed between the inner face of the housing and the outer face of each billet to introduce the billet into the housing.
This is why actual extrusion is preceded by a precompression operation during which the billet, as it starts to be crushed by the two closing pieces moving towards each other, slightly expands and is fully applied against the inner face of the housing of the container.
The air in the original clearance between the billet and inner wall of the housing is thus highly compressed and must be expelled by a "degassing" operation before extrusion.
In forward extrusion processes, this degassing following precompression is normally achieved by commanding a slight backward movement of the container with the ram to allow the container to detach from the bottom carrying the die, thereby allowing the compressed air in the housing to escape. To this end, the press is provided with auxiliary jacks for isolated maneuvers of the container, and jacks for pushing back the main extrusion jack, which is normally a single-acting jack.
These operations call for the reversal of hydraulic commands by acting on pumps, valves or other elements, with relatively long response times. This results in dead time that reduces the productivity of the press and which up to now has seemed unavoidable since inadequate degassing can lead to defects such as bubbles or blisters in the extruded product and even introduce a risk of explosion during extrusion.